Drop use of the deprecated drmP.h file from drm/nouveau.

Build tested using allyesconfig and allmodconfig.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>